I will try not to be too picky but perhaps you could try re-reading some of the stuffs said. Some things just don't seem right or a bit confusing. For example:
So yes, the economy is actually a major factor in having a good economy.
I believe you either meant to say:
"So yes, the economy is actually a major factor in having a good server."; or
"So yes, the population is actually a major factor in having a good economy."
But I do get what you are trying to say mostly - I just need to try to think a bit what you might actually mean.
Adding on to your Guide:Non-People FactorSome servers introduce additional NPC items (stems, poison spores, witch star sands, red gems, yellow gems, blue potions, grape juice, alcohol, EDP poison bottle, etc... ) which also form part of the supply and do affect the economy. On the other side of the equation, almost everything on RO has an NPC price associated with it (which may or may not be a fair price) and could contributes to the demand. So, those who buys and sells items may not necessary be "People".
CorruptionsHaving an older server with a larger populations do comes with their own problems pertaining to economy. I have seen and heard of many servers which have many corruptions- GM spawning items for their Guilds members, friends or themselves or just dropping them in the streets of Prontera, bug abuse, duplication of accounts, zeny, items, characters etc.... all actions which negatively affects the economy. While they may not be facing similar problems as a newly startup server, they could face the situation where their economy is flooded with too many items and zeny (mostly of questionable nature). You may also have heard of some of the more common solutions such as server wipes or zeny sinking.
Ageing PlayersHaving an older server with a larger population do not necessary guarantee people actually hunting items. Many veterans I know just log on RO for two hours each week for WOE only. And you see them next WOE. They are pretty self-sufficient. They have already amassed millions/billions of zeny throughout their years playing and have already the attained multiple sets of Godly gears and rare items. There's nothing worth buying. (No Demand) And they don't need to sell anything to make more zeny either (No Supply). Well - they don't really demand and supply much; that's what I am trying to say. They may buy some healing items or other WOE supplies. And for all we know, these items could might as well be just NPC items; which bypass the "People" entirely.
Information AsymmetryLack of information for players to decide and agree on a fair price of an item. Creating a mismatch in pricing. Buyers wants a lower price. Sellers wants a higher price. How do you tell how much is an item? Do you have past historical records of trading prices? In the most extreme situation, new players (or ill-informed players) will be ripped off.
Buyer: Hey, you must be new here?
Seller: Yes I am. My first week. I got this item - it's an Alice card. What shall I do with it?
Buyer: Oh, it's useless. It's just for collections. As a matter of fact, I am missing that one piece to complete my collections. If you look carefully, the NPC price for that card is only 10z. But since you are new, I am willing to pay you 10 times that amount = 100z. How does that sounds?
Seller: Cool. Thanks. Here you go, take this Alice card.
Concentration of WealthOver time, players leave the server, some may give their stuffs away to their closest friends/guildies . Imagine a server with a peak population of 3,000; now only with less than 50 active veteran players. Imagine the concentration of wealth in those 50 players. And how they could influence the economy.
CliquesOvertime, players will cement strong bonds among themselves - some friendship other more than just friends (partners/family/etc..) So, you need 6 Megingjards for WOE/Leveling with your friends? Sure, I will lend them to you. Just keep them until u are finished with them- no rush. I think I have 10 more in the 30+ accounts I now own. That's what good friends/family do. Well the action of sharing is very noble, but the economic implications could be no demand for quest items needed to make Megingjards. If you want one, just borrow from a friend. Well, if you are new and don't belong to a clique - tough luck.